Generation of pyroclastic flows by explosive interaction of lava flows with ice/water-saturated substrate

Abstract

We describe a new type of secondary rootless phreatomagmatic explosions observed at active lava flows at volcanoes Klyuchevskoy (Russia) and Etna (Italy). The explosions occurred at considerable (up to 5 km) distances from primary volcanic vents, generally at steep (15–35°) slopes, and in places where incandescent basaltic or basaltic-andesitic lava propagated over ice/water-saturated substrate. The explosions produced high (up to 7 km) vertical ash/steam-laden clouds as well as pyroclastic flows that traveled up to 2 km downslope. Individual lobes of the pyroclastic flow deposits were up to 2 m thick, had steep lateral margins, and were composed of angular to subrounded bomb-size clasts in a poorly sorted ash–lapilli matrix. Character of the juvenile rock clasts in the pyroclastic flows (poorly vesiculated with chilled and fractured cauliflower outer surfaces) indicated their origin by explosive fragmentation of lava due to contact with external water. Non-juvenile rocks derived from the substrate of the lava flows comprised up to 75% in some of the pyroclastic flow deposits. We suggest a model where gradual heating of a water-saturated substrate under the advancing lava flow elevates pore pressure and thus reduces basal friction (in the case of frozen substrate water is initially formed by thawing of the substrate along the contact with lava). On steep slope this leads to gravitational instability and sliding of a part of the active lava flow and water-saturated substrate. The sliding lava and substrate disintegrate and intermix, triggering explosive “fuel–coolant” type interaction that produces large volume of fine-grained clastic material. Relatively cold steam-laden cloud of the phreatomagmatic explosion has limited capacity to transport upward the produced clastic material, thus part of it descends downslope in the form of pyroclastic flow. Similar explosive events were described for active lava flows of Llaima (Chile), Pavlof (Alaska), and Hekla (Iceland) indicating that this type of explosions and related hazard is common at snow/ice-clad volcanoes and sometimes happens also on fluid-saturated hydrothermally altered slopes.